Generally, rare-earth magnets are commonly used in motors and generators because they allow the motors and generators to be more compact in size. However, the supply of rare-earth magnets is not sustainable due to limited availability and skewed supply locations. Therefore, it is desirable to reduce the amount of rare-earth magnets in motors and generators.
For example, a patent document 1 (i.e., Japanese Patent Laid-Open No. 2010-252530) discloses a consequent-pole type rotor in which a permanent magnet is positioned within every other pole. That is, when the first and second salient poles are alternatingly arranged in a circumferential direction, a permanent magnet is only positioned in the first salient pole. A soft magnetic material pole is positioned in the second salient pole. In such a consequent-pole type rotor, the soft magnetic material pole is “consequently” magnetized to have the same pole as an inner face side pole of the permanent magnet that is buried in the magnetic pole, which results in an alternating arrangement of N poles and S poles on the rotor's surface. As a result, the number of permanent magnets used in the rotor is reduced in half, thereby simultaneously reducing cost and rare-earth magnet procurement risk.
However, a consequent-pole type rotor may suffer from cogging torque due to interaction between the permanent magnets. As a result, cogging torque may produce torque output fluctuations at low speeds and reduce motor efficiency.
With regards to cogging torque, a distribution of magnetic flux in a gap between the rotor and the stator may change according to a balance between a first magnetic resistance at an inside portion in the radial direction and a second magnetic resistance at an outside portion in the radial direction relative to the permanent magnet in the rotor. More practically, when reducing the second magnetic resistance by shifting the position of the permanent magnet toward an outside in the radial direction, the magnetic flux density between the magnetic pole of the rotor and the teeth on the stator increases. Alternatively, when increasing the second magnetic resistance by shifting the position of the permanent magnet toward an inside in the radial direction, the magnetic flux density between the magnetic pole of the rotor and the teeth on the stator decreases. Especially, in a buried magnet type rotor, which generally has a smaller gap between the rotor and the stator, the balance between the first magnetic resistance and the second magnetic resistance increasingly affects the magnetic flux density.
Therefore, a correlation exists between (i) a dimension of a certain part that is related to the position of the permanent magnet in the radial direction and (ii) the output torque and the cogging torque, and completed the present disclosure.